Functions of the Large Intestine

Unlike the small intestine, which secretes digestive enzymes and absorbs the products of digestion, the large intestine has little or no digestive function. However, the mucous membrane that forms the inner lining of the large intestine contains many tubular glands. Structurally, these glands are similar to those of the small intestine, but they are composed almost entirely of goblet cells. Consequently, mucus is the only significant secretion of this portion of the alimentary canal (fig. 17.49).

Mechanical stimulation from chyme and parasympa-thetic impulses control the rate of mucus secretion. In both cases, the goblet cells respond by increasing mucus production, which, in turn, protects the intestinal wall against the abrasive action of materials passing through it. Mucus also holds particles of fecal matter together, and, because it is alkaline, mucus helps control the pH of the large intestinal contents. This is important because acids are sometimes released from the feces as a result of bacterial activity.

Chyme entering the large intestine usually has few nutrients remaining in it and mostly consists of materials

Lumen

Mucosa

Figure 17.47

The rectum and the anal canal are located at the distal end of the alimentary canal.

Lumen

Mucosa

Submucosa

Muscular layer Serosa

Figure 17.48

Light micrograph of the large intestinal wall (64x).

Submucosa

Muscular layer Serosa

Figure 17.48

Light micrograph of the large intestinal wall (64x).

Goblet cells

Light micrograph of the large intestinal mucosa (250x micrograph enlarged to 560x).

Light micrograph of the large intestinal mucosa (250x micrograph enlarged to 560x).

not digested or absorbed in the small intestine. It also contains water, electrolytes, mucus, and bacteria.

Absorption in the large intestine is normally limited to water and electrolytes, and this usually occurs in the proximal half of the tube. Electrolytes such as sodium ions can be absorbed by active transport, while the water follows passively, entering the mucosa by osmosis. As a result, about 90% of the water that enters the large intestine is absorbed, and little sodium or water is lost in the feces.

The many bacteria that normally inhabit the large intestine, called intestinal flora, break down some of the molecules that escape the actions of human digestive enzymes. For instance, cellulose, a complex carbohydrate in food of plant origin, passes through the alimentary canal almost unchanged, but colon bacteria can break down cellulose and use it as an energy source. These bacteria, in turn, synthesize certain vitamins, such as K, B12, thiamine, and riboflavin, which the intestinal mucosa absorbs. Bacterial actions in the large intestine may produce intestinal gas (flatus).

O How does the structure of the large intestine differ from that of the small intestine?

^9 What substances does the large intestine absorb?

^9 What useful substances do bacteria inhabiting the large intestine produce?

Hirschsprung disease causes extreme, chronic constipation and abdominal distension. The part of the large intestine distal to the distension lacks innervation. As a result, the person does not feel the urge to defecate. The problem begins in the embryo, when an abnormal gene prevents neurons from migrating to this portion of the gastrointestinal tract. Surgery may be used to treat Hirschsprung disease, which was once lethal.

Goblet cells

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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